To comply with the increasingly more restrictive standards aimed at curtailing atmospheric pollution caused by exhaust gases of motor vehicles, it is necessary to implement a purification system called SCR (“Selective Catalytic Reduction”) in the exhaust system of the motor vehicles themselves, in particular of freight and passenger transportation vehicles, “duty trucks” and “heavy trucks”.
The SCR system consists in injecting an aqueous solution of 32.5% of urea by compressed air at the catalyser inlet, located at the exhaust gas muffler: in the catalyser, the urea reacts with the nitrogen oxides contained in the exhaust gases, eliminating such oxides from the atmospheric emissions.
To accomplish this process it is necessary therefore to make available the urea solution in a tank, from where the solution itself can be taken to be injected into the catalyser, after having possibly been mixed with the compressed air, according to times and methods defined by an electronic control unit which accounts for various external (temperature, humidity, etc.) and internal (engine operation, number of revolutions, etc.) parameters.
It is necessary that the urea solution flows in the entire range of motor vehicles working temperatures, that is at temperatures from −40° C. to +80° C., and in all flow rate conditions (from 0 to 5.5 l/h).
However, the urea solution is serviceable only in liquid phase, and its freezing temperature is equal to approximately −11° C. Therefore, if the conveying means remains at temperatures under this value, the SCR system must be equipped with electrical heated pipings.
For this purpose, in general, the piping of SCR systems are provided with an external spirally wound electrical resistor. In use, while vehicles equipped with the SCR system are parked, the urea solution freezes at temperatures lower than −12° C.: when the engine is ignited after parking, the pipings containing the urea solution are immediately heated by passage of electrical current.
In practice, it is felt the need to reach a temperature of at least +5° C. for the urea solution in less than ten minutes after engine ignition, with the SCR system initially placed at an ambient temperature of −35° C.
The known solutions described above are not very satisfactory, because they cannot always achieve such a need and eliminate “plugs” of solid solution which are formed inside the hydraulic pipe fittings connecting the heated pipings to the other components of the SCR system.
Indeed, the heat generated by the electrical resistors spirally wound on the pipings cannot always sufficiently warm up the solidified part of solution inside the pipe fittings.